feat: OG image sizing and responsive cover srcsets (#447)
* feat: add OG image sizing and responsive srcsets for cover images - Resize og_image to 1200x630 (Fill) in opengraph template with og:image:width/height meta tags for proper social preview rendering. - Replace single 1200x resize with 800x400 card + 1600x800 retina srcset in article-cover template for responsive cover images. Closes #358 Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> * fix: use JPEG for OG images and derive dimensions from resource Address Copilot review: WebP has spotty social preview support; switch to JPEG q85 for OG images. Derive width/height from the processed resource instead of hardcoding to prevent drift. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> --------- Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>